I have a set of the Swift Tires ( black wall ). They measure 30mm at 45psi on VERY narrow rims. That works out to just a bit under 1 1/4" . The O.D. on these rims is 20mm sooo maybe 18mm I.D. ? The cheap 1 1/4 tires I took off were measuring around 27mm. I'm sure they would be plumper on normal rims.
I am very happy with them. Smooth, fast, and enough tread to last a while. Definite step up from the Chin Shens they replaced.